1. Field of the Invention
This invention relates to switchgear used in low voltage electric power distribution systems, and more particularly, to an arrangement for channeling arc gases generated by an electrical switching apparatus such as a circuit breaker incorporated into the switchgear, and also for guiding and protecting from the arc gases control and instrumentation wiring for the electrical switching apparatus.
2. Background Information
Switchgear for electric power distribution systems includes electrical switching apparatus and their line and load terminations together with related equipment mounted in metal cabinets. Switchgear used in sections of electric power distribution systems operating at voltages up through 600 volts is classified as low voltage switchgear. Typically, the electrical switching apparatus is a circuit breaker, but other switching apparatus such as for instance, network protectors, disconnect switches and transfer switches are also mounted in such switchgear cabinets. Henceforth, the electrical switching apparatus will be referred to as circuit breakers, although it will be understood that other types of electrical switching apparatus can be used as well.
Typically in such low voltage switchgear, multiple circuit breakers are mounted in each cabinet in cells stacked vertically in a forward compartment. The line and load conductors are mounted in rearward compartments and engage the circuit breakers through quick disconnects as the circuit breakers are installed in the cells.
Power circuit breakers can generate significant amounts of arc gases when interrupting large currents such as those associated with a short circuit in the distribution system. It is common for the arc gases to be vented through the rear of the circuit breaker. Other power circuit breakers discharge the gases through vents in the top of the circuit breaker housing. Generally, the approach is to slow the arc gases down and cool them before they leave the switchgear cabinet.
Other considerations in the design of low voltage switchgear is the placement of the terminals for the secondary wiring, such as the control and instrumentation wiring. Generally, there is not much room for these secondary wiring terminations within the circuit breaker cabinet. One approach is to terminate the secondary wiring in the rear of the switchgear cabinet. However, this can expose personnel tending the secondary terminations to power conductors which are located there.
It is preferable to have the secondary wiring terminations at the front of the switchgear cabinet both for safety and accessibility. Hence, one approach has been to mount the secondary terminations alongside the circuit breakers. However, such space is very limited in the typical switchgear cabinet which is of standardized width.
Another approach has been to use an entire cell for terminations. Obviously this limits the number of circuit breakers that can be mounted in a switchgear cabinet. In some switchgear, the secondary terminations are mounted on trays which slide between the vertically mounted cells. In one arrangement, the trays pull out horizontally and then swing vertically for better access. One non-domestic manufacturer has fixed mountings for secondary terminations between the cells.
Placing the secondary terminations between the circuit breakers and accessible from the front of the cabinet must include protection of wiring from the arc gases discharged from top vents on the breakers. The non-domestic manufacturer referred to above utilizes a rectangular channel extending transversely above the top vents to deflect the arc gases out sideways. The fixed secondary terminations are then mounted separately in front of the transverse channel.
The switchgear utilizing the pull out trays have a flat, horizontal arc shield at the top of the cells above the top vents. In some switchgear, this type of flat horizontal shield also extends under the pull out tray which is forward of the top vents. In this arrangement, where the pull out tray swings vertically after being pulled out, the flat wall is lower under the tray and there is an inclined transition section to this lower forward section. However, this is well in front of the arc vents. The switchgear also has flanges extending downward along the side edges of the flat ceiling panel.
There is a need for improvement in low voltage switchgear for containing the arc gases generated by the newer power circuit breakers with top arc vents and higher interruption ratings, and for safe and convenient secondary terminations which are protected from the high volumes of the arc gases.
This need and others are satisfied by the invention which is directed to an arc shield/wire tray for secondary wiring terminations for electrical switching apparatus having arc chute vents in an upper surface thereof and mounted in a switchgear cabinet. The arc shield/wire tray comprises an upward and rearwardly inclined rear panel positioned over the arc chute vents for deflecting arc gases rearward. A floor panel extends forward from a front edge of the inclined rear panel and a front wall extends upward from a front edge of the floor panel. Secondary terminations provided on terminal blocks are mounted to the front wall for access from the front of the cabinet. Where the electrical switching apparatus has secondary wiring connectors on an upper surface forward of the arc chute vents and forward of the inclined rear panel, the floor panel of the wire tray has a cut-out for the secondary wiring connectors.
When used in a switchgear cabinet having a vertical passage for wiring, the arc shields/wire trays open on one end for communicating with this vertical passage. This opening can be provided in a side wall of the arc shield/wire tray.
Preferably, the secondary terminations are mounted in the front panel in at least two substantially horizontal rows with a lower row positioned farther forward than an upper row. This arrangement is provided by the front wall having an upper vertical section and a lower vertical section and a connecting section positioning the lower vertical section further forward than the upper vertical section. When used in switchgear cabinets having a plurality of vertically stacked cells in which the electrical switching apparatus is mounted, the arc shield/wire tray is suspended from a cell above.
The invention also embraces a wire tray for secondary wiring for electrical switching apparatus mounted in vertically aligned cells in a switchgear cabinet in which the wire tray has a rear wall, a bottom wall and a front wall each extending substantially across the cell and means mounting terminations for the secondary wiring in the front wall. Where the electrical switching apparatus has secondary wiring connectors at a top surface, the bottom wall of the wire tray has a cut-out for the connectors and for passing wiring from the connectors to the terminations. The terminations are mounted in the front wall in two substantially horizontal rows with the lower row forward of the upper row. Where the switchgear cabinet has a vertical wiring passage, the wire tray communicates at one end with this vertical passage. In a preferred arrangement, the wire tray has side walls extending between the rear wall and the front wall and an opening in one of the side walls for communicating with the vertical wiring passage. Where the switchgear cabinet has a plurality of vertically stacked cells, the wire trays are suspended from a cell above.
The invention is also directed to switchgear apparatus for housing electrical switching apparatus having arc chute vents spaced rearwardly in upper surfaces thereof. A cabinet having a forward compartment for the electrical switching apparatus has an additional compartment behind the forward compartment. At least one cell in the forward compartment houses an electrical switching apparatus and an arc shield mounted over the cell has an arc deflecting member positioned above the arc chute vents and inclined upward and rearwardly to deflect the arc gases ejected through the arc vents rearward. Preferably, the arc shield has an integral downwardly extending lip positioned rearwardly of the arc deflecting member. More preferably, the arc shield has an integral substantially horizontal section extending between the arc deflecting member and the downwardly extending lip. Also, preferably, the cabinet has a vertical passageway extending along at least one side of the arc deflecting member into which the arc gases escape by moving laterally from the downwardly extending lip.
Where the switchgear apparatus has multiple cells stacked vertically in the forward compartment, arc shields for a plurality of the cells are suspended from a cell above. In a preferred arrangement, at least one cell has a wire tray extending across the cell above the electrical switching apparatus and forward of the arc shield. This wire tray utilizes the arc deflecting member as a rear wall. It further has a bottom wall extending forward of the arc deflecting member, a front wall forward of the bottom wall and secondary terminal blocks supported in the front wall and accessible from a front of the cabinet.
Where the electrical switching apparatus has quick disconnects extending rearwardly therefrom below the downwardly extending lip on the arc shield, a quick disconnect protector extends between quick disconnects and the downwardly extending lip to protect the quick disconnects from the arc gases. Preferably, this quick disconnect protector is a generally horizontally extending perforated plate.